Purge and trap is a well known technique for the extraction (purging or sparging) of chemicals out of liquids (including water). In a typical laboratory method, an inert gas, often helium, is used to transfer chemicals from the liquid phase to the gas phase. Once in the gas phase the chemicals are transferred to a variety of traps such as an adsorbent bed or liquid nitrogen cold trap.
Several commercial portable systems provide for detection of water-borne volatiles. One such system, the Inficon Scentograph CMS 200 gas chromatograph, is a portable field gas chromatograph that can be powered in a DC mode by rechargeable batteries. For fixed-location operations, the unit can be connected to an AC source. The unit includes an internal supply cylinder for a carrier gas, in particular argon for a MAID/ECD configuration or helium for a photoionization detector (PID) or thermal conductivity detector (TCD) configuration. The instrument further comprises a GC module and a detector module. The GC module includes an oven with two stage temperature ramping capability, columns, and detector(s). Several different detectors are available for different applications. An argon ionization detector (AID) provides detection of organic compounds having an ionization potential of 11.7 eV or below, including halomethanes and haloethanes. This detector purportedly is capable of detecting these compounds, as well as other hydrocarbons, down to low ppb levels. A micro argon ionization detector (MAID) is a small volume, higher sensitivity version of the AID, that is used with capillary columns with detection limits purportedly at the sub ppb levels. An electron capture detector (ECD) is a selective detector used for the detection of halogenated hydrocarbons, PCB's, pesticides, and nitro-based compounds. A thermal conductivity detector (TCD) is used primarily for the detection of natural gases in concentrations ranging from 100 ppm to percent levels. A photoionization detector (PID) uses an ultraviolet lamp to ionize and detecthydrocarbons having ionization potentials of 10.6 eV or less.
A drawback of the Inficon Scentograph CMS 200 gas chromatograph and similar instruments is high cost and the requirement for an on-board carrier gas.
Also known is the DR 2800 Portable Spectrophotometer manufactured by Hach Company that uses a colorimetric system that can detect THM chemicals. This system does not speciate THM species but instead provides one concentration value for over four chemicals. The system uses multiple chemical packets that an analyst has to open and add to the water sample.
Still another instrument is the ZNOSE® analyzer available from Electronic Sensor Technology. The ZNOSE® Model 4200 is a portable real time analyzer that can detect and analyze vapors and identify traces of organic, biological and chemical compounds. As understood, the analyzer is a surface acoustic wave (SAW) detector-based system. For volatiles in water, the analyzer performs a headspace analysis. An optional desorption tube collection device is not portable and requires AC power. An optional water sample heater is also not portable and requires AC power.